Transmission of television video signals is sometimes momentarily or temporarily interrupted, such interruptions being referred to in the art by the term "glitch". Glitches may be caused by a variety of factors, inter alia, environmental factors such as lightning or weather changes, interference from buildings, electrical/electronic failures during transmission, or satellite communication problems, such as poor placement of satellite dishes or temporary transmission or reception failures. In the case of encrypted transmissions which are deciphered by a decryption key, system "bugs" or failure may prevent reception of the decryption key.
Glitches may be perceived by a viewer as a shaking, blurring, fuzziness, halting, etc., of the picture on the television screen. In subscriber satellite television transmission, glitches can cause further problems. In such subscriber systems, an encrypted television signal is transmitted together with a conditional access stream which contains entitlement control messages (ECM's) which comprise, inter alia, decryption key information. Encrypted systems are described, for example, in U.S. Pat. Nos. 5,282,249 and 5,481,609 to Cohen et al. and assigned to the present assignee, the disclosures of which are incorporated herein by reference. An integrated receiver decoder (IRD) receives the encrypted signal and uses the decryption key information to decode the ECM's which determine whether the viewer is authorized to view the particular broadcast. If a glitch occurs in such an encrypted system, particularly a glitch in the ECM stream, the IRD is unable to interpret the signal and momentarily no picture at all is transmitted to the television set. Depending on the programming of the IRD, the momentary non-transmission of a picture may be perceived as a frozen or blue screen or a scrambled picture.
It is thus readily understood that detecting glitches is an important tool in providing reliable broadcasting service. By detecting glitches, it may be possible to discover their source and eliminate or at least cover up and/or mend the problem.
Glitch detectors are known in the art. U.S. Pat. No. 4,107,651 to Martin describes a glitch detector and circuit for detecting glitches upon a digital signal depending upon the digital signal level and the polarity of subsequent transitions within a discrete sampling period. U.S. Pat. No. 4,198,608 to Comley describes a glitch detector and trap circuit for removing a glitch generated by a D/A converter due to an error in one or more bits of an input digital signal. U.S. Pat. No. 4,353,032 to Taylor describes a system that employs two glitch detectors associated with complementary data signals that can detect either positive-going or negative-going glitches within a sample clock period. U.S. Pat. No. 4,495,621 to Nakagomi et al. describes a glitch detector which recognizes and measures the lengths of glitches by sampling an input signal at fractions of a sampling clock.
Published PCT patent application WO 96/34294 describes a method for non-invasively testing performance of a digital communication system. The test system takes advantage of information typically generated by digital receivers to correct for communication channel imperfections.
Other published patent documents attempt to detect glitches by transmitting a known test pattern which is viewed at the receiving end, any glitches in the received test pattern being readily detectable. An example of such art is Published PCT patent application WO 95/24101 which describes apparatus for creating video test patterns for outputting to a video test pattern generator, which test pattern is displayed to an operator in picture representation which simulates the appearance of a test pattern when displayed on a television receiver. European Patent Application EP 729280 A2 describes a transmission monitoring system which receives a test signal from a given source and provides a feedback indicative of the received test signal to the source via a standard voiceband telephone connection. EP 746168 A1 describes a test pattern generator which generates a test pattern that can be visually inspected by a test engineer to determine whether or not certain lines of the test pattern are being properly decoded by a television signal decoder. EP 746169 A1 describes a test pattern suitable for a television signal decoder which detects changes in the color content of a signal from frame to frame, and on the basis of these changes modulates certain parameters, such as attenuation of high frequency illuminance components. EP 757499 A2 describes a method of testing a compressed digital television signal decoder with a signal analyzer that checks whether the decoder provides an expected signature of bits from a test bit stream.
However, the above cited prior art does not provide a glitch detector for automatically detecting glitches in a test pattern that can efficiently be used for subscriber satellite television transmission.